1. Field of the Invention
This invention relates to a center-pole type linear motor and, more particularly, to a center-pole type linear motor operating on three-phase AC current.
2. Prior Art
Linear motors are in general use in several technologies. In transportation technologies, for example, the magnetic levitation train, linear motors can be utilized as transportation vehicles. In other technologies, linear motors are employed to position printer heads, as variable position actuators and for linear positioning of other systems. Linear motors are commonly used, for example, in micro-lithographic instruments for positioning objects such as stages, and in other precision motion devices for precisely controlling the position of devices and instruments.
Linear motors are described in Hazelton et al., U.S. patent application Ser. No. 09/059056, "Linear Motor Having Polygonal Shaped Coil Units", filed Apr. 10, 1998, and assigned to the same assignee as the present disclosure, herein incorporated by reference in its entirety. A conventional linear motor includes a magnet array, which creates a magnetic field, and a coil array. The linear motor generates electromagnetic forces (called Lorentz forces) on the coil array in cooperation with the magnet array. The electromagnetic forces on the coil array cause the coil array to be propelled with respect to the magnet array. Conventional linear motors may incorporate a stationary magnet array (where the coil array is propelled) or a stationary coil array (where the magnet array is propelled). A translation stage is commonly attached to the coil array so that the translation stage, and therefore any object that is attached to the translation stage is propelled by the linear motor.
A conventional moving coil type linear motor comprises permanent magnets disposed on both sides of a moveable coil. Typically, the permanent magnets are permanently fixed on the inside surfaces of two rails, i.e., the surface of each rail that is directed towards the other rail. The coil array is typically mechanically coupled to a carriage (or translation stage) that is slideably engaged with a set of rails, which can be the two side rails or some other set of rails. The coils themselves are wound in a direction perpendicular to the magnetic fluxes of the magnetic field created by the permanent magnets. By externally generating currents in the coil array, the carriage will experience a Lorentz force and, consequently, will be propelled.
FIGS. 1A and 1B show respectively a plan and endview of a center-pole type linear motor 100 having two side rails 101 and 102. Side rails 101 and 102 are parallel plates of magnetic material that run the length of linear motor 100. End rails 103 and 104 are engaged with side rails 101 and 102 at both ends of linear motor 100. A base plate 113 (FIG. 1B) is attached to side rails 101 and 102 to form a "U" shaped yoke 111. Two permanent magnets 105 and 106 are mounted on the inside walls of side rails 101 and 102. Magnets 105 and 106 are magnetized along their thicknesses and mounted with the same polarity against the inside walls of yokes 101 and 102 (e.g., with the south pole directed towards the yoke as shown in FIG. 1A). A center pole 107 is mounted in air gap 114 between magnets 105 and 106 and runs parallel to side rails 101 and 102 through the length of linear motor 100. A coil bobbin 108 is attached to a carriage 112 (FIG. 1B) and displaced around center pole 107. Coils 109 and 110 are wound around coil bobbin 108 in a direction perpendicular to the line of travel of coil bobbin 108 (i.e., along the length of linear motor 100). Alternatively, flat coils may be wound in a direction parallel to the line of travel of coil bobbin 108 and fastened to coil bobbin 108 in such a fashion as to intersect the flux lines between either of magnets 105 and 106 and center pole 107. Flat coils, however, are problematic in that they have a larger percentage of inactive wire in comparison with coils wound around center pole 107. In either case, the principle that a current applied to the coils will cause the coils and their coil bobbin 108 (which is attached to carriage 112), to experience a force and, therefore, to move relative to the magnets, applies.
Base plate 113 is of non-magnetic material, such as 304 stainless steel, aluminum or ceramic. Side rails 101 and 102, end rails 103 and 104 and center pole 107 are of magnetic material (e.g., iron or steel) typically with saturation flux density equal to or greater than about 16,000 gauss. Permanent magnets 105 and 106 are, for example, of high quality neodymium iron boron (NdFeB) permanent magnet material with a residual permanent magnetic flux density of about 13,500 gauss or greater. Permanent magnets 105 and 106 are typically coated to prevent corrosion.
Linear motor 100 shown in FIGS. 1A and 1B has several deficiencies, including that the concentration of magnetic flux in end rails 103 and 104 and at the ends of center pole 107 causes magnetic saturation of those materials or, if more material is used to accommodate higher magnetic fluxes, causes linear motor 100 to become massive. In addition, the force felt by coil bobbin 108 in response to a given input current varies as coil bobbin 108 travels through linear motor 100. As a result, centerpole-type linear motors of this type suffer from a limited range of motion and a varying force over the range of motion. The only way to increase the range of motion of the linear motor is to increase the length of permanent magnets 105 and 106, which is not easily achievable.
FIG. 2 shows in a plan view a linear motor 200 similar to that described in U.S. Pat. No. 4,318,038, "Moving-Coil Linear Motor", issued to Hidehiko Munehiro on Mar. 2, 1982. In linear motor 200, side rails 201 and 202 are parallel plates that run the length of linear motor 200. End rails 203 and 204 are engaged with side rails 201 and 202 at both ends of linear motor 200. A bottom plate (not shown) is attached to side rails 201 and 202 to form a "U" shaped yoke. Center pole 207 is a parallel plate that runs, in parallel with side rails 201 and 202, the length of motor 200. Two (2) coils, coil 209 and coil 210, are mounted on a bobbin 208 that is slideably attached to center pole 207. Coils 209 and 210 are wound in a direction perpendicular to the direction of travel of bobbin 208. Permanent magnet segments 205-1 through 205-N are mounted to the inside surface of side rail 201 (i.e., the side of yoke 201 towards center pole 207) and magnet segments 206-1 through 206-N are mounted to the inside surface of yoke 202. Magnet segments 205-1 through 205-N and 206-1 through 206-N are individual magnets having alternating polarities and width approximately equal to twice the length of each of coils 209 and 210. The effect of including permanent magnet strips having alternating polarity is that the magnetic material in center pole 207, end rails 203 and 204 and side rails 201 and 202 are not so easily saturated.
However, coils 209 and 210 are actuated in a full on or full off fashion and therefore suffer from a non-constant force. This form of commutation results in a non-constant force along the length of motor 200.
It is desirable to have a linear motor that is extendible to any length without suffering magnetic saturation in the materials. In addition, it is desirable to have a linear motor capable of applying, to a moving stage, a substantially constant force along its entire length.